Infrared imaging optical systems are typically used to view and image light energy in the infrared optical spectrum. The production of infrared light is typically associated with the production or release of heat by hot objects such as engines and living mammals, such as for example human beings. Infrared energy is capable of transmission through many conditions that would otherwise block visible light, such as clouds of particulate matter, water vapor, vegetation covering, and various forms of optical camouflage.
Missiles fired at an aircraft may be detected by the heat and corresponding infrared signatures produced by their engines, regardless of whether the missile is guided by an active or passive targeting system. Aircraft that are potentially targets for such missiles may carry infrared warning devices that view the exterior world in search of heat signatures that are associated with the engines of such missiles. Upon the detection of such a missile, such systems provide advance warning to the aircraft pilot and crew. In one type of infrared-warning device, fixed infrared warning sensors are positioned at locations on the target aircraft. The sensors include an array of lenses that focus the external infrared energy onto a cryogenically cooled detector. The detector converts the incident infrared energy to electrical signals, which are analyzed for infrared signatures that may be associated with threats to the aircraft such as fired missiles.
The infrared warning sensors may use an inverse telephoto lens system, sometimes termed a “fisheye” lens, because it allows the field of view to be very large. The image gathered by such a fisheye lens is typically provided to an infrared detector, such as a focal plane array (FPA). The focal plane array is typically an array of small pixels, each being operable to produce an electrical signal in response to infrared radiation being incident upon the pixel.
Inverse telephoto optical systems are widely available for the visible spectrum. For infrared optical systems, however, many fewer types of inverse telephoto optical systems are available because the infrared detector must be cryogenically cooled. The lenses of the optical system are preferably not cooled, because a very large, high-capacity cryostat would be required and because the cooling from room temperature to the cryogenic operating temperature would alter the positions of the lenses due to thermal expansion/contraction. The inverse-telephoto lens system should therefore have an external pupil for the location of a cold shield that surrounds only the cryogenically cooled detector and not the lenses.